Linda's new book is The Center of Everything and so lovely, just like everything she writes. Hound Dog True is excellent, too. She's very natural.
Here's an oldie but goodie, but I love everything Noel Streatfeild and she does wonderful dialogue for characters of all ages (adults right down to the smallest kid in the family). You can even "hear" their regional/cultural accents without it being too much. Warning: the characters are British, so it will make you want some tea.
Ditto E. Nesbit. I love it when people write siblings talking like siblings actually talk. Very old-fashioned stuff, but I love it.
Actually, I'm starting to think that great dialogue comes in books where people have very natural-acting characters, behaving and speaking in a way that rings very true and not writerly.